1. Field of the Invention
The present invention relates to units, especially glazing, with variable optical/energetic properties. It relates more precisely to glazing some of whose characteristics can be modified under the effect of an electrical input, for example the light diffusion or the transmission of electromagnetic radiation at some wavelengths, especially in the infrared or in the visible.
2. Discussion of the Background
An increasingly keen demand exists, in fact, for so-called "intelligent" glazing, that is to say glazing whose properties can be modulated at will, especially in order to take into account various parameters that are capable of changing progressively. Thus, it may be found highly advantageous to be able to control sunlight input through glazing fitted externally in buildings or vehicles of the motor vehicle or train type, in order to prevent excessive heating of the rooms or compartments in the event of strong sunshine. Similarly, it may be useful to be able to control the degree of vision through glazing, especially to reduce it or even to prevent it completely for a certain time, for example in the case of glazing employed as internal partitions between two rooms, in a building, or between two compartments, in a means of locomotion of the train or aircraft type. Many other applications also exist for such glazing: it is possible, for example, to mention vehicle rear-view mirrors which, by becoming darker in case of need, can prevent dazzling of the driver, or road or urban sign boards showing messages or designs only intermittently, so as better to attract attention. Transparent glazing units can also be mentioned which can have a sufficiently diffusing state to be employed as projection screens.
The interest which such glazing arouses justifies the fact that many systems have already been studied. Thus, known systems which make it possible to modulate the light transmission or absorption of glazing are especially so-called viologen systems, as described in U.S. Pat. No. 5,239,406 or in patent EP-A-0 612 826.
For the same purpose there are also so-called electrochromic systems, the principle of operation of which is briefly recalled: in a known manner, these comprise a layer of an electrochromic material capable of inserting cations and electrons reversibly and simultaneously and whose oxidation states corresponding to the inserted and deinserted states are of different colour, one of the states exhibiting a higher light transmission than the other. The insertion or deinsertion reaction is control-driven by a suitable electrical input with the aid of a current generator or a voltage generator. The electrochromic material, usually tungsten oxide-based, must thus be placed in contact with a source of electrons, such as a transparent electrically conductive layer and with a source of cations, such as an ion-conductive electrolyte.
Furthermore it is known that, in order to ensure at least a hundred switchovers or so, the layer of electrochromic material must be used in combination with a counterelectrode, also capable of inserting cations reversibly, symmetrically in relation to the layer of electrochromic material, such that, macroscopically, the electrolyte looks like a simple medium for the cations.
The counterelectrode must consist either of a layer of neutral colour, or at least transparent or weakly coloured when the electrochromic layer is in the faded state. Since tungsten oxide is a cathodic electrochromic material, that is to say that its coloured state corresponds to the most reduced state, an anodic electrochromic material such as nickel oxide or iridium oxide is generally employed for the counterelectrode. It has also been proposed to employ a material which is optically neutral in the oxidation states concerned, like, for example, cerium oxide or organic materials like electron-conductive polymers (polyaniline etc.) or Prussian blue.
The description of such systems will be found, for example, in European patents EP-0 338 876, EP-0 408 427, EP-0 575 207 and EP-0 628 849.
At present these systems can be classified into two categories, depending on the type of electrolyte which they employ:
either the electrolyte is in the form of a polymer or of a gel, for example a proton-conductive polymer such as those described in European patents EP-0 253 713 and EP-0 670 346, or a polymer conducting lithium ions, such as those described in patents EP-0 382 623, EP-0 518 754 or EP-0 532 408,
or the electrolyte is an inorganic layer, ion-conductive but electronically insulating; these are then referred to as "all solid" electrochromic systems. For the description of an "all solid" electrochromic system, reference may be made to the French patent application filed on Mar. 27, 1996 under the filing number FR-96/03799.
The viologen or electrochomic systems deposited or associated with transparent substrates constitute glazing whose light absorption and transmission (as well as energy transmission) can vary within given ranges, ranges that are determined especially by the choice of the electrochromic materials employed and/or by the choice of their thickness. However, this glazing exhibits some limits which are intrinsic to it: thus, even though some configurations allow it to attain very low light transmission values in the coloured state, for example of the order of 1% down to 0.1%, it cannot ensure complete obscuring, in other words, its light transmission level is, despite everything, still too high to prevent the identification of an object or of a person through the glazing by an observer placed on the other side of the said glazing, above all if there is a stronger light source on the side of the object, for example in the case of a room lit at night.
To obtain this effect the electrochromic glazing alone cannot therefore suffice, and it will be necessary to add to it an external unit which, depending on requirements, will either make vision blurred through the glazing, like a net curtain, or will simply prevent vision completely by opacifying the latter, this being the case of a unit of the shutter type. However, these units require maintenance, age badly, and have properties which cannot be modulated.
Another type of "intelligent" glazing consists of what is referred to by the term of optical valve: this is a film including a generally crosslinked polymer matrix in which are dispersed microdroplets containing particles which exhibit the property of adopting a preferred direction under the action of an electrical or magnetic field.
The film exhibits variable optical properties, especially as a function of the potential applied to the terminals of the conductive layers placed on either side of the film and of the concentration and nature of the orientable particles.
Thus, patent WO-93/09460 discloses an optical valve based on a film including a crosslinkable polyorganosilane matrix and inorganic or organic orientable particles, more particularly particles that absorb light, such as polyiodide particles. When a voltage is applied to the film, the particles intercept light much less than when it is without voltage.
A glazing whose operating principle is similar is also known under the name of liquid-crystal glazing. It is based on the use of a film placed between two conductive layers and based on a polymeric material in which are dispersed droplets of liquid crystals, especially nematic with positive dielectric anisotropy. When a voltage is applied to the film, the liquid crystals are oriented along a preferred axis, and this permits vision. With no voltage, in the absence of crystal alignment, the film becomes diffusing and prevents vision.
Examples of such films are described especially in European patent EP-0 238 164 and U.S. Pat. Nos. 4,435,047, 4,806,922 and 4,732,456. Film of this type, once laminated and incorporated between two glass substrates, is marketed by Saint-Gobain Vitrage under the trade name "Priva-lite".
It is possible, in fact, to employ any devices containing liquid crystals known under the names of "NCAP" (Nematic Curvilinearly Aligned Phases) or "PDLC" (Polymer Dispersed Liquid Crystal).
It is also possible to employ, for example, polymers containing cholesterol liquid crystals, like those described in patent WO-92/19695.
This glazing itself also has its limitations: thus, while actually making it possible to obtain a variable light diffusion, it does not make it possible to darken significantly the room or the compartment in which it can be fitted.
In order also to successfully modulate light transmission, there is a solution which consists in adding dyes, of the dichroic dye type, to the film. However, two disadvantages remain: on the one hand, with these dyes the film exhibits some instability, very particularly to ultraviolet radiation, and these dyes are soluble not only in the droplets of liquid crystals, but also partially in the polymer; this restricts the possibilities of modulating the light transmission of the system. Furthermore, the changes in light diffusion and transmission are closely dependent on one another: it is possible to obtain only high light transmission/nondiffusing state or low light transmission/diffusing state "pairs".